Claude Ghez, M.D.

• Reflex and voluntary movements are sensorimotor transformations.
– Feedforward vs. feedback control.
• Reflex control differs from voluntary control
– Spatial organization of reflexes reflects hard-wired connections vs. behavioral demands and context: decisions. Voluntary movements are organized to be appropriate to address behavioral goal: kinematic vs. dynamic transformations - internal models.
– Reflex latency & duration reflect mainly fixed neuromuscular constraints: conduction, transmission, contraction. These, also influence timing of voluntary movement, but information processing and accuracy constraints are the critical reaction time and movement duration.
– Neural organization of voluntary movements is highly dependent on learning and plasticity. Adaptability is critical over long term.

1. Voluntary movements require decisions and information processing: Latency and duration:
• Reaction time.
• Parallel processing.
• Speed-accuracy tradeoffs.
2. Sensorimotor transformations in reaching and grasping.
• Kinematics: visuomotor transformations. Movement vectors
• Dynamics: internal models.
• Role of vision and proprioception in feedback and feedforward control
3. Organization of motor cortical areas for reaching and grasping
• Multiple motor areas
• Somatotopic organization
• Redundancy

Latency or “Reaction time” depends on decision
Reaction time paradigm
• Warning -> cue= go signal
•Simple RT: single or predictable cue Subject knows what response to make in advance
•Choice: multiple unpredictable cues (e.g. colors, symbols, spatial locations) each requiring different responses.
Stage theory of reaction time

“Reaction time” depends on practice and learning
Unpredictable stimuli
Predictable sequence

Response features can be processed in parallel
Synch Cue
Tones
Elbow force
Measured by
Strain gauge
Target
Resting force
>20
0
100-
200
High
Middle
Low
Low
Middle
High
<100
High
Middle
Right direction
Low
Low
Middle
Wrong
High direction

SpeedAccuracy tradeoff (Fitts’ law)
Error varies with speed

Reaching reflects several sensorimotor transformations kinematic and dynamic planning

Reaching: hand and joint kinematics are planned independently

Extent and direction are planned in advance
Paths are straight Hand trajectories:
Speed & acceleration
Scale with distance
Reaching involves scaling a ‘trajectory primitive’ to target distance.
Learned scaling factors
& reference frame

Accuracy requires knowledge of mechanical properties of the limbs (“the plant”): role of proprioception
Normal control Patient without proprioception
Mobility

Extent
Directional variations in inertial resistance are corrected by compensatory variations in movement time
Proprioception is critical
Control MFG Patient MA
PK Acceleration
300
Movement time (ms)
0
Peak
Acceleration
Movement Direction
900
0
Peak
Acceleration
Movement Direction

Proprioceptive information is used for feedforward control: Internal models

Reaching reflects several sensorimotor transformations
Kinematics and dynamics
Parallel planning of concurrent actions: hand preshaping

Motor areas of the brain
Macaque Monkey Human
Corpus Callosum
Premotor cortex
Supplementary motor area (SMA)
Primary motor cortex
Supplementary motor area (SMA)
Primary motor cortex
Central sulcus
Premotor cortex
Primary sensory cortex
Posterior Parietal
Cortex
Prefrontal cortex

Somatotopic organization revieled by electrical stimulation of the cortical surface

Electrical stimulation is medically useful
Early experiments

Electrical stimulation is medically useful:
More recent experiments

Representation of individual muscles is patchy and distributed
Deltoid Ext. Carpi Rad.
Posterior
Anterior
Medial
Posterior
Lateral
Anterior
Medial Lateral

Cell activity associated with movement of individual digits is broadly distributed: convergence of spinal projections